1. Field of the Invention
This invention relates to methods of laying seabed pipelines from a surface vessel.
The invention is particularly, although not exclusively, applicable to laying pipelines in deepwater locations. Such pipelines are usually formed from lengths of steel pipe having a corrosion resistant coating such as an epoxy coating and are joined together by welding or snap connectors. It is not usually necessary to provide such pipelines with concrete coatings for protection because of the depth of the seabed.
2. Background Prior Art
In the production and laying of seabed pipeline, for the transportation of oil or gas from a vessel onto the sea bottom three basic methods are commonly used:
1. S-lay
The pipeline leaves the layvessel in a substantially horizontal orientation, bends downwards over a supporting structure, the so-called stinger, and when approaching the sea bottom bends upwards to be laid on the sea bottom. The sideview of the pipeline being laid shows an S, the shape dependant on pipe weight, water depth and the tension applied to the conduit. For a typical example see U.S. Pat. No. 3,715,890.
The advantage of this method is its relatively high production speed in case the conduit is composed of steel pipes to be welded together and the vessel is long enough to deploy an economic number of welding stations. The disadvantage, however, is that for very deep water either the tension to be applied to the conduit will become too high or a stinger of such a size is required that it is not feasible anymore to connect such a stinger to the vessel.
2. J-Lay
The pipeline leaves the lay vessel in an inclined or even near vertical orientation in order to allow the laying of pipelines in deep water without stressing the pipeline material excessively. This method is explained in U.S. Pat. Nos. 3,266,256 and 3,389,563. The method can also be used in shallow water as illustrated in an article entitled "The one-ship work fleet", published, in Ocean Industry of March 1970, pages 52-54.
A particular disadvantage of this method is the relatively slow production speed because it is often only possible to work in only one welding station which reduces the production speed significantly compared with the S-lay method where welding is done in a number of welding stations.
3. Reel-lay
Reel-lay can be considered as a variation on J-lay because the conduit leaves the lay vessel in an inclined or even near vertical orientation but in reel-lay a length of pipeline is held in a storage reel to be unreeled at site. This method gives a very high lay speed and is obviously advantageous when a flexible pipeline or cables have to be laid. Examples of this method can be found in U.S. Pat. No. 3,389,563, FIG. 4 and U.S. Pat. No. 4,340,322 which describe a self-propelled reel pipe laying ships. A disadvantage of this method relates to the limited capacity of the reels, necessitating regular reloading or change out of reels when a long pipeline is being laid.
Ideas to overcome some disadvantages related to the above mentioned lay methods and combining the advantages of the same are given in WO 95.25237. To limit the bending radius of the steel pipe and hence to limit the diameter of the reel or other curved guiding means, a certain plastic deformation of the pipeline is allowed. This normal procedure when using the reel-lay method however requires expensive straightening means to straighten the pipeline after this member has left the reel or any other curved part having introduced the plastic deformation of the pipeline. Other disadvantages of the proposed methods in WO 95/25237 are the necessity to bend and straighten the pipeline more than once (shown in FIGS. 4 and 5) or that a certain fleet angle is required to allow the vertical part of the pipeline to pass the horizontal part of the pipeline.